The present invention relates to a liquid crystal display apparatus using a liquid crystal modulation element, such as a liquid crystal projector.
Some of the liquid crystal modulation elements are realized by putting nematic liquid crystal having positive dielectric anisotropy between a first transparent substrate having a transparent electrode (common electrode) formed thereon and a second transparent substrate having a transparent electrode (pixel electrode) forming pixels, wiring, switching elements and the like formed thereon. The liquid crystal modulation element is referred to as a Twisted Nematic (TN) liquid crystal modulation element in which the major axes of liquid crystal molecules are twisted by 90 degrees continuously between the two glass substrates. This liquid crystal modulation element is used as a transmissive liquid crystal modulation element.
Some of the liquid crystal modulation elements utilize a circuit substrate having reflecting mirrors, wiring, switching elements and the like formed thereon instead of the abovementioned second transparent substrate. This is called a Vertical Arrangement Nematic (VAN) liquid crystal modulation element in which the major axes of liquid crystal molecules are alignment in homeotropic alignment substantially perpendicularly to two substrates. The liquid crystal modulation element is used as a reflective liquid crystal modulation element.
In these liquid crystal modulation elements, typically, Electrically Controlled Birefringence (ECB) effect is used to provide retardation for a light wave passing through a liquid crystal layer to control the change of polarization of the light wave, thereby forming an image from the light.
In the liquid crystal modulation element, which utilizes the ECB effect to modulate the light intensity, application of an electric field to the liquid crystal layer moves ionic materials present in the liquid crystal layer. When a DC electric field is continuously applied to the liquid crystal layer, the ionic materials are pulled toward one of two opposite electrodes. Even when a constant voltage is applied to the electrodes, part of the electric field applied to the liquid crystal layer is cancelled out by the charged ions to substantially attenuate the electric field applied to the liquid crystal layer.
To avoid such a phenomenon, a line inversion drive method is typically employed in which the polarity of an applied electric field is reversed between positive and negative for each line of arranged pixels and is changed in a predetermined cycle such as 60 Hz or the like. In addition, a field inversion drive method is used in which the polarity of an applied electric field to all of arranged pixels is reversed between positive and negative in a predetermined cycle. Those drive methods can avoid the application of the electric field of only one polarity to the liquid crystal layer to prevent the unbalanced ions.
This corresponds to controlling the effective electric field to be applied to the liquid crystal layer such that it always has the same value as the voltage to be applied to the electrodes.
The variations of the effective electric field applied to the liquid crystal layer, however, are caused not only by the abovementioned movement of the ionic materials but also by other factors. One of the other factors causes trapping of charges of electrons or holes in a non-conductive film such as a liquid crystal alignment film made of an insulating material, a reflection enhancing film, and an inorganic passivation film for preventing dissolution of metal. The trapping causes charge-up on the interface of the film, and that electrostatic charge changes the effective electric field applied to the liquid crystal layer with time.
The charging phenomenon may be seen due to the shape in the transmissive liquid crystal modulation element and occurs prominently in the reflective liquid crystal modulation element including opposite electrodes formed of different materials (mirror metal and indium tin oxide (ITO) film).
To avoid the charging phenomenon, the following technique has been disclosed in Japanese Patent Laid-Open No. 2005-49817. In the method disclosed therein, a work-function adjusting film layer is formed on a reflecting pixel electrode to control the work function of the reflecting electrode to be ±2% or less relative to the work function of a transparent electrode (ITO film electrode) opposite thereto, thereby reducing charge-up on an interface layer of the liquid crystal to avoid occurrence of flicker or image sticking on the liquid crystal modulation element (or the liquid crystal display apparatus with the same).
In addition, trapping of charges requires excitation hopping of the energy potential of the insulating film. In Japanese Patent Laid-Open No. 2005-49817, the probabilities of the excitation hopping from the metallic mirror electrode and the ITO transparent electrode are made substantially equal to each other, thereby generating charge-up due to charge trapping of the same amount on both electrode sides.
This results in a shift in potential of the electric field applied to the liquid crystal layer in the field inversion drive method whereas a change in magnitude of the electric field does not occur. Since the electric field generated in the liquid crystal depends on the relative value between the opposite electrodes, the operation of the liquid crystal does not change.
However, only providing a film for adjusting the difference of the work functions between the opposite electrodes to the liquid crystal modulation element is not sufficient to ensure reliability thereof in a long term. The charges charged up in the liquid crystal layer are gradually accumulated with operating time of the liquid crystal modulation element, and thereby the potential difference between the mirror electrode and the ITO electrode reaches a few hundred millivolts in operating time from a few thousand hours to a few tens of thousand hours. This phenomenon occurs more often as the photon energy entering the liquid crystal modulation element and total amount of light energy increase.
The potential difference between the mirror electrode and the ITO electrode causes the difference of retardation modulation of liquid crystal depending on the polarity of the electric field applied on the liquid crystal layer, and thereby the light modulation intensity oscillates at 60 Hz in the case of driving at 60 Hz by the field inversion drive method. The oscillation of the light intensity at 60 Hz cannot be sensed by human eyes.
When the amplitude of the oscillation increases such that the potential difference between the opposite electrodes exceeds more than 200 mV, a low frequency component of the oscillation increases, thereby causing a large oscillation of the light intensity which is visible to a human eye as flicker. The visibility thereof is high particularly when a 50 percent intensity modulation is performed in which a gradation gamma changes drastically.
Furthermore, the potential difference between the mirror electrode and the ITO transparent electrode due to the charge-up on the liquid crystal interface layer causes an additional problem. Specifically, the constant DC electric field is continuously applied to the liquid crystal layer, so that ionic materials present in a small amount in the liquid crystal layer is pulled toward one of the opposite electrodes. The ionic material may be pulled toward the interfaces on both sides of the liquid crystal layer depending on the polarity of the charge of the ion.
Since the ions attached to the interface of the electrode are moved in accordance with the amplitude of a drive potential in the field inversion drive, the attachment state of the ions varies with the level of the amplitude of the drive potential. This results in variations of the effective electric field applied to the liquid crystal layer at different positions in a display area, which causes sticking. When the same image is displayed for a long time and then a different image is displayed, the previous image is seen as an afterimage. This is called the image sticking (or simply, sticking).